1. Field of the Invention
The present invention relates to an anode material including tin and a battery using the anode material.
2. Description of the Related Art
In recent years, portable electronic devices such as cellular phones, camera/VTR (video tape recorder) combination systems, laptop computers or the like have come into widespread, and a reduction in size and weight of the devices have been strongly required. Accordingly, as portable power sources for the devices, research and development aimed at improving the energy density of batteries, specifically secondary batteries have been actively promoted. Among the batteries, lithium-ion secondary batteries hold great promise, because the lithium-ion secondary batteries can obtain a higher energy density than lead-acid batteries or nickel-cadmium batteries which are conventional aqueous electrolyte secondary batteries.
Conventionally, as an anode material of the lithium-ion secondary battery, a carbonaceous material such as non-graphitizable carbon, graphite or the like is widely used, because the carbonaceous material exhibits a relatively high capacity and excellent charge-discharge cycle characteristics.
However, in accordance with a recent trend toward a higher capacity, an anode is required to have a further higher capacity, so research and development has been promoted. For example, an anode including a carbonaceous material has achieved a higher capacity by selecting the carbonaceous material and a forming method (refer to Japanese Unexamined Patent Application Publication No. Hei 8-315825). However, the anode has a discharge potential of 0.8V to 1.0 V against lithium, so when a battery is formed using the anode, the battery discharge voltage becomes lower. Therefore, a significant improvement in a battery energy density is not expected. Further, there are disadvantages that a hysteresis in charge-discharge curves is large, and energy efficiency in each charge-discharge cycle is low.
On the other hand, as an anode material capable of achieving a higher capacity, for example, a material adapting a property of reversibly producing and decomposing some kind of lithium metal by an electrochemical reaction. More specifically, as such a material, a Li—Al alloy has been known since long ago. Further, a Si alloy has been reported (refer to U.S. Pat. No. 4,950,566).
However, these anode materials such as these alloys are expanded or shrunk in accordance with charge and discharge, thereby resulting in the materials being pulverized, so the cycle characteristics of the battery declines.
Therefore, in order to improve the charge-discharge cycle characteristics, an anode material in which an element not involved in expansion and shrinkage in accordance with insertion and extraction of lithium is substituted for a part has been studied. For example, LiSiaOb (0≦a, 0<b<2) (refer to Japanese Unexamined Patent Application Publication No. Hei 6-325765), LicSi1-dMdOe (M represents metal except for alkali metal or metalloid except for silicon; 0≦c, 0<d<1, 0<e<2) (refer to Japanese Unexamined Patent Application Publication No. Hei 7-230800), and a Li—Ag—Te alloy (refer to Japanese Unexamined Patent Application Publication No. Hei 7-288130) have been invented.
However, even if any of these anode materials is used, a decline in the cycle characteristics resulting from expansion and shrinkage of the alloys is large, so the fact is that full advantage cannot be taken of a higher capacity.
Moreover, D. Larcher et al. has proposed Cu6Sn5 which is an intermetallic compound as an anode material capable of achieving a higher capacity. However, in Cu6Sn5, a large decline in the charge-discharge cycle characteristics is still observed.